The buses were targeted as they were passing through Dahyan market in the rebel-held province of Saada, according to the Houthi-run Al-Masirah TV.

"A total of 43 people, mostly pupils under the age of 10, were killed and 64 were injured when two Saudi-led airstrikes hit passenger buses in Dahyan popular market," Yahya Shayem, head of Saada Health Office, told Xinhua news agency.

"The victims were mostly pupils who were on their way to attend a summer school in Dahyan," he said.

The International Committee of the Red Cross (ICRC) said a hospital it supported in Saada had received dozens of casualties after the attack.

The Saudi-owned Al Arabiya television quoted a statement by the Saudi-led coalition as saying that "Thursday's airstrikes targeted Iranian-allied Houthi rebels who fired a ballistic missile on Wednesday at the Saudi commercial city in border Jazan region."

The coalition, backing Yemen's government in a war with the Houthis, said its actions were "legitimate", according to the BBC.

In response, Houthi spokesman Mohammed Abdulsalam called the coalition statement "absolutely ridiculous".
"They killed pupils driving to school ... this is a war crime," Abdulsalam tweeted.

The Houthi spokesman's comments came at the same time the coalition warplanes hit the Houthi-controlled capital Sanaa 11 times, with no reports of casualties yet. The airstrikes targeted Sabeen Square and two military camps.

Houthi media aired gruesome footage appearing to show the bodies of children in school uniform. Other footage showed a young boy being escorted to a hospital, with blood all over his face.

The ICRC stressed that "under international humanitarian law, civilians must be protected during conflict", while the Secretary-General of the Norwegian Refugee Council Jan Egeland called it a "grotesque, shameful" attack that showed "blatant disregard for rules of war".

It was not immediately clear whether the bus was the target of airstrikes, but coalition spokesman Col. Turki al-Malki said the attack was "a legitimate military action, conducted in conformity with international humanitarian law".

A military coalition led by Saudi Arabia has been carrying out airstrikes since 2015 against Houthi positions -- causing thousands of civilian deaths in the process, according to war monitors -- in an attempt to restore the internationally-recognized government of President Abdrabbuh Mansur Hadi.